It will take a while for you to be able to travel to another campaign but get to Cantha as the quests there give alot of exp and the henchies are lvl 20.
I actually wen't outside of nundu bay to the castle whete acolyte of melandru is and killed stuff there doing a few quests along the way and it leveled me up pretty fast (I was also a dervish)...it was fast from 1-14, then took a while to get to about 17, and then it shot by to 20 again...the biggest delays between levels for me were somewhere around the mission where you first meet abbadon...